Method of determining material using intensity of light

ABSTRACT

A profiling method compensates for phase changes associated with the presence of multiple or varying material in the area to be measured. The profiling method measures at least a portion of the height profile of the area of interest. The phase of the different materials in the region are also obtained and used to generate a correction factor. Depending on the type of material in the region of interest, the correction factor may be the material specific phase difference of the materials in the region, e g., when at least one of the materials is opaque to the wavelength of light used to measure the height profile, or the relationship between the thickness and phase of the material for a desired thickness range, e.g., when one or more of the materials is transparent to the wavelengths used to measure the height profile. The correction factor is then used to correct and/or convert the measured phase profile to an actual height profile. Accordingly, an accurate height profile may be obtained for regions that include dishing, erosion, or that contain various types of materials.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.09/71,813, filed Nov. 28, 2000 now U.S. Pat. No. 6,633,359, entitled“Profiling Method”.

FIELD OF THE INVENTION

The present invention relates to measuring the surface profileproperties of features. The proposed metrology method compensates forphase changes associated with the presence of multiple or varyingmaterials on the surface of a substrate and in particular, to ametrology method to measure dishing, erosion, and/or an actual heightprofile of features on a sample.

BACKGROUND

The metal interconnect of integrated circuits has conventionally beenrealized by blanket depositing a layer of metal on a planar insulatingsurface. Portions of the metal layer are subsequently removed in aphotolithographically patterned etching step to form the resulting metalconductors. Conventional integrated circuits have generally employedsomewhat resistive metal, such as aluminum, or metal alloys for themetal interconnect. Copper has been chosen as a replacement metal foraluminum in smaller geometry devices. Due to complexities associatedwith etching copper, it must be patterned in a different manner. Copperis blanket deposited over the wafer that has trenches and vias etchedinto the dielectric and then it is subjected to chemical mechanicalpolishing (CMP) to remove the copper from the upper planar surface. Thegoal is to have a globally planar surface composed of copper anddielectric regions.

FIGS. 1A through 1G show a cut-away view of the conventional fabricationof an aluminum interconnect. As shown in FIG. 1A, a relatively planarsurface layer 10, which may be, e.g., a silicon substrate, is coveredwith a dielectric layer 12, e.g., an oxide layer, which is patterned andetched. An aluminum layer 14, which may be an aluminum alloy, is blanketdeposited over the dielectric layer 12, as shown in FIG. 1B. Aphotoresist layer 16 is deposited over the aluminum layer 14 (FIG. 1C),and is exposed and developed resulting in the structure shown in FIG.1D. The aluminum layer 14 is then etched, e.g., using a plasma etchingtechnique, resulting in the structure shown in FIG. 1E. The remainingphotoresist layer 16 is removed resulting in the structure shown in FIG.1F. After these steps are completed, the surface is composed of metallines with near vertical sidewalls above the surface of the dielectriclayer 12, as shown in FIG. 1F. Subsequently, dielectric layers aredeposited and etched over the metal lines to yield a dielectric layer 18with a planarized surface, e.g., for the next metal layer, as shown inFIG. 1G.

A major change is being implemented in semiconductor processing byswitching from aluminum to copper metallization. Copper is preferred toaluminum due to its lower resistivity and better electromigrationresistance. Unfortunately, copper is difficult to etch and the switchfrom aluminum to copper has forced a change in the basic metallizationprocess. Copper cannot simply be substituted for aluminum in themetallization process because plasma etching of copper is more difficultthan plasma etching of aluminum (due to the lack of volatile copperhalogen compounds). Additionally, if copper is allowed to directlycontact the dielectric materials, it can rapidly diffuse throughdielectric materials and contaminate the semiconductor devices.

Thus, a “damascene” process has been developed whereby copper can beused as the interconnect metal. Rather than blanket depositing theinterconnect metal on a substantially planar insulating substrate andthen etching away parts of the metal layer to leave the conductors,trenches are formed in an insulating material. A composite layer of adiffusion barrier, nucleation layer and copper are then blanketdeposited over the entire surface of the insulating substrate such thatthe trenches arc filled. Chemical mechanical polishing is then used toplanarize the integrated circuit surface and thereby polish away all themetal that is not in the trenches. The result is metal conductorsdisposed in trenches and a globally planarized surface.

FIGS. 2A through 2C show a cut-away view of the conventional fabricationof a copper interconnect. As shown in FIG. 2A, a relatively planarsurface layer 50, which may be, e.g., a silicon substrate, is coveredwith a dielectric layer 52, e.g., an oxide layer, which is patterned andetched. The dielectric layer 52 may be patterned and etched in multiplesteps in order to produce trenches 54 and via 55. A diffusion barrierlayer (not shown), nucleation layer (not shown), and copper layer 56 areblanket deposited over the dielectric layer 52 such that the trenches 54and via 56 are filled, as shown in FIG. 2B. A chemical mechanicalpolishing step is then used to planarize the surface of the copper layer56 (along with the diffusion barrier layer and nucleation layer) withdielectric layer 52, resulting in the structure shown in FIG. 2C.

The ideal copper CMP process removes the copper, nucleation layer anddiffusion barrier from the surface of the dielectric while leavingbehind the copper, nucleation layer and diffusion barrier in thetrenches and contacts or vias. The ideal result would be a globallyplanarized surface with no vertical height change over the entire wafersurface. FIG. 3 shows the ideal resulting structure with a planarsurface composed of a dielectric region 52 and idealized copper region56. Global planarity is desirable because of the depth of fieldrequirements associated with the lithographic steps. Significant heightvariations on the surface will compromise the photoresist processingsteps and subsequently the etching and metallization processes. Heightvariations are also symptomatic of undesirable variations in the copperthickness and metal line resistance.

Unfortunately, because of the complexities associated with the CMPprocess, global planarity is not achievable. An artifact of the CMPprocesses in copper metallization results from the copper and dielectricmaterial having different polishing rates, resulting in what is known as“dishing.” FIG. 4 shows a cut-away side view of the typical resultingstructure after the CMP process, in which the surface of the copperregion 56 a is lower than the surrounding dielectric region 52 a. Itshould be understood that FIG. 4 is for exemplary purposes and is not toscale. Dishing may generally be defined as the maximum height differencebetween the metal region 56 a and the adjacent dielectric region 52 aafter CMP processing.

Another artifact caused by the CMP process, as known to those ofordinary skill in the art, is “dielectric erosion,” i.e., the dielectricregions exhibit a change in height over the surface of the wafer. Thisvariation is related to the local density of metal features. Areascontaining no metal features exhibit the highest dielectric surfaces,areas of low metal density exhibit relatively high dielectric surfaceregions and areas of high metal density result in relatively lowdielectric surface regions.

The processing of silicon wafers to form integrated circuit chipsrequires many complex processing steps, for example, those describedabove. Each step must be carefully monitored and controlled to maximizethe quality and yield of the final product. With the imminentreplacement of aluminum by copper to form the metallization layers onsilicon wafers, new processes and metrology techniques must be developedand implemented to characterize the degree of surface planarizationafter the CMP step.

Accordingly, what is needed is an economical, reliable, rapid, preciseand accurate metrology procedure that can characterize and controlindividual steps during processing of a sample and specifically thatwill can be used to measure dishing, erosion, curvature, and/or theactual height profile of features on the sample.

SUMMARY

A profiling method, in accordance with the present invention,compensates for phase changes associated with the presence of multiplematerials or materials, such as transparent or composite materials,having varying thickness in an area of a substrate to be measured. Thephase profile of the area of interest is first measured, e.g., using adifferential interferometer. If there is more than one material present,the constant material specific phase shift associated with an opaquematerial or the thickness dependent, material specific phase associatedwith a transparent or composite material at a single location areobtained. For each pair of materials hit by the reference andmeasurement spots, the difference in the phase values for the twomaterials is used to generate a phase correction factor for theappropriate fraction of the data. Next, the phase versus thicknessrelationship is generated for any transparent or composite materialsover the thickness range of interest. The phase versus thicknessrelationship is used to convert the measured phase to actual thicknessor height for the transparent or composite regions. The phase versusthickness relationship for an opaque material is constant, so nocorrection is required for opaque regions. When all of the data isappropriately corrected, the present invention advantageously generatesan accurate thickness or height profile for regions on a sample that mayinclude dishing, erosion or other surface features in the presence ofmore than one material or stack of transparent materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1G show a cut-away view of the conventional fabricationof an aluminum interconnect.

FIGS. 2A through 2C show a cut-away view of the conventional fabricationof a copper interconnect.

FIG. 3 shows a cut-away view of the ideally planar metal and dielectricregions resulting from a planarization process.

FIG. 4 shows a cut-away view of the typical resulting structure after aCMP process showing dishing of the metal region.

FIG. 5 shows a top view of the starting and ending locations of thereference and measurements spots associated with a referential scan ofan opaque feature surrounded by transparent material.

FIG. 6 shows a plot of the relative phase profile (lateral distance vs.relative phase) generated by the referential scan in FIG. 5.

FIG. 7 shows a plot of the actual height profile (lateral distance vs.actual height) generated by the referential scan in FIG. 5 aftercorrecting for phase errors.

FIG. 8 shows a top view of the starting and ending locations of thereference and measurements spots associated with a differential scan ofan opaque feature surrounded by transparent material.

FIG. 9 shows a plot of the relative phase profile (lateral distance vs.relative phase) generated by the differential scan in FIG. 8.

FIG. 10 shows the detector voltage signal as a function of themodulation voltage from a differential interferometer measurement.

FIG. 11 shows a cut-away view of the height profile associated with afeature in a single opaque material.

FIG. 12 shows a cut-away view of the height profile associated with afeature composed of one opaque material surrounded by a second opaquematerial.

FIG. 13 is a flow chart describing the process of generating the actualheight profile of a sample composed of two opaque materials using areferential scan.

FIG. 14 is a flow chart describing the process of generating the actualheight profile of a sample composed of two opaque materials using adifferential scan.

FIG. 15 shows a cut-away view of the height profile associated with afeature composed of an opaque material surrounded by transparentmaterial.

FIG. 16 is a flow chart describing the process of generating the actualheight profile of a sample composed of an opaque material and atransparent material using a referential scan.

FIG. 17 is a flow chart describing the process of generating the actualheight profile of a sample composed of an opaque material and atransparent material using a differential scan.

FIG. 18 shows the plot of the relationship of the absolute phase and thethickness for the transparent region from FIG. 15.

FIG. 19 shows a cut-away view of the thickness profile associated withthe transparent material immediately adjacent to the opaque feature inFIG. 15.

FIG. 20 shows a cut-away view of the thickness profile associated with acomposite feature including both opaque and transparent material andthat is surrounded by transparent material.

FIG. 21 shows the top view of the composite feature of FIG. 17intercepted by the measurement spot.

FIGS. 22A and 22B are a flow chart describing the process of generatingthe actual thickness profile of a sample composed of a transparentmaterial and a composite material using a referential scan and twomaterial specific phase measurements.

FIGS. 23A and 23B are a flow chart describing the process of generatingthe actual thickness profile of a sample composed of a transparentmaterial and a composite material using a differential scan and twomaterial specific phase measurements.

DETAILED DESCRIPTION

A metrology process, in accordance with the present invention, directlymeasures the profile of a surface region of a substrate that may includeone or more materials, e.g., two opaque materials, one opaque and onetransparent material or two transparent materials. It should beunderstood that the terms opaque and transparent refer to the materialbeing opaque or transparent to the wavelengths of light being used tomeasure the profile of the surface region. An opaque material may be,e.g., a metal, metal alloy, mixture of metals, extremely thicktransparent material or mixture of extremely thick transparentmaterials. A signal will be present at the surface of the opaquematerial from a reflection from the top surface but not from areflection from an underlying interface of the opaque material withanother material. A transparent material may be a dielectric material ora composite material with a transparent component. A composite materialwith a transparent component may be made up of any combination of opaqueand transparent materials or multiple transparent materials. Thedistribution of the two or more components in the composite materialwill typically have a characteristic dimension orthogonal to themeasurement beam that is smaller than the spot size of the differentialinterferometer, otherwise the material would be treated as two discreetmaterials. The phase response from a composite material will thus be afunction of the properties of the two or more materials and the geometryassociated with the composite material. A signal will be present at thesurface of the transparent or composite material from a reflection fromthe top surface as well as from one or more phase shifted reflectionsfrom one or more underlying interfaces.

The method corrects the measured phase profile using a phase correctionfactor and/or a method of converting phase to thickness for transparentor composite regions. The present invention may be used to generateheight or thickness profiles of regions on semiconductor wafers, flatpanel displays, or other similar flat substrates. The present metrologyprocedure, for example, can quantify dishing after a planarizationprocess, e.g., a CMP process, in a fast, precise, accurate, reliable andeconomical manner.

In accordance with the present invention, the height profile of aregion, e.g., the actual height difference between the opaque andadjacent transparent regions, is directly measured. Direct measurementof the surface height of opaque features, e.g., metal lines, andtransparent features, e.g., dielectric regions, with a form of radiationis difficult because these two materials respond to radiation in acomplex manner. Metal features are essentially opaque to most forms ofradiation and produce a constant material specific phase shift, whilethe dielectric material is partially transparent to most forms ofradiation that are used for measurement purposes and also modifies thephase response in a complex manner. Layers or features buried in thedielectric layer may also affect the reflected signal making theanalysis of the dielectric surface more difficult. Thus, anunderstanding of the complex modifications of the phase shift of thedifferent materials is necessary, and may be obtained using databases ofknown information and/or using detailed measurements, e.g., with anellipsometer.

The presence of more than one material or one or more transparent orcomposite materials will introduce phase errors when the profiling tooluses radiation that interacts with the material. The present inventionuses an optical tool, e.g., a differential interferometer, to measurethe phase profile of a region and includes a method that compensates forthese measured phase errors and can be used to correct the measuredphase profile to produce an actual height or thickness profile.Consequently, an accurate measurement of dishing, erosion, curvature orany other desired height or thickness profile of interest may beobtained.

As is well known in the art, a differential interferometer, creates twospots with orthogonal polarization states. The sample to be measured maybe scanned by the differential interferometer in two different manners:in a reference mode and a differential mode. FIG. 5 illustrates the useof a differential interferometer in a reference mode, where one spot(the reference spot) 102 stays at two or more points, a line or an areaof constant thickness or height. If desired, the reference spot 102 maystay at only one point, but that is practically difficult withconventional differential interferometers. The second spot (themeasurement spot) 104 scans two or more points, a line or an area of theregion of interest, which includes, e.g., feature 106 surrounded by area108, which is composed of a different material compared to feature 106.FIG. 6 illustrates the resulting relative phase profile 110 versusscanning distance plot, which is related to the actual height profile112, shown in FIG. 7, except for phase related errors. It is typicallyadvantageous to have a relatively large spacing of the two spots 102 and104 in the reference mode. The phase values are typically plottedrelative to the first point of the scan because the phase valuesgenerated by the differential interferometer contain a constant offsetrelated to properties associated with the optical path and are thusmeaningless.

FIG. 8 illustrates the use of a differential interferometer indifferential mode, where the reference spot 102 and the measurement spot104 follow the same path with, e.g., the measurement spot 104 beingahead of the reference spot 102. FIG. 9 illustrates the resulting plot114, which is closely related to the derivative of the relative phaseprofile 110 in FIG. 6. If the resulting plot 114 is numericallyintegrated, the relative phase profile 110 in FIG. 6 is generated. Therelative phase profile 110 of FIG. 6 may need to be corrected for phaseerrors to generate the actual height profile 112, shown in FIG. 7. Thephase values are once again typically plotted relative to the firstpoint of the scan. It is theoretically possible to generate the samerelative phase profile from either a referential or differential scan ifthe differential scan is numerically integrated. Both methods mayrequire phase corrections to produce the actual or thickness profile.

When two or more materials are present during a differentialinterferometer scan, one must know the material being intercepted byeach spot. This is required for the appropriate conversion of themeasured phase profile to the corrected phase profile to the actualheight profile. The signal from the laser is phase modulated, split intotwo beams, bounced off of the sample, recombined back into a singlebeam, made to interfere through a polarizer and measured by a detector.The resultant signal at the detector is in the form of a sine wave (FIG.10). A phase or height measurement is associated with each sine wave.The hardware and principles involved are described in G. Makosch and B.Solf, “Surface Profiling by Electro-optical Phase Measurements”, SPIEVol. 316 High Resolution Soft X-Ray Optics, (1981), which isincorporated herein by reference. In this example of a singlemeasurement point, the signal was modulated for slightly more than onefull wavelength. The voltage intensity from the detector can be analyzedusing the following equation:

Intensity=A+B×Cos(CV+D)  eq. 1

where A is the average intensity of the detector signal, B is one halfof the peak to peak intensity, C is the modulator sensitivity, V is theapplied modulator voltage and D is the phase shift associated with thetwo measurement spots. For each sinusoidal wave from the detector, A andB can be used to determine the intensity of each spot using thefollowing formulas: $\begin{matrix}{{I1} = \frac{A + \sqrt{A^{2} - B^{2}}}{2}} & {{eq}.\quad 2} \\{{I2} = \frac{A - \sqrt{A^{2} - B^{2}}}{2}} & {{eq}.\quad 3}\end{matrix}$

At any wavelength used by the differential interferometer, 11 and 12will vary with the material being intercepted by each spot. The changein 11 and 12 as the scan progresses can be used to determine thespecific materials being intercepted by each spot. For example, whencopper and a dielectric material are present, the amplitude of thesignal from the copper surface is approximately twice as large as thesignal from the dielectric material. Prior knowledge of the amplitudeassociated with different materials is therefore required. Theinformation relating amplitudes with different materials can be storedin a database to be used as needed. When a spot intercepts twomaterials, the amplitude will be determined by the area weighted averageof the two materials. Typically, the phase response when two materialsare intercepted is not analyzed. This is not of great consequence sinceit is typically a small fraction of the data.

In the rare case of two materials exhibiting similar intensities,additional information is required. For example, one might know that thehigher portion of a step is associated with one material and the lowerportion of the step is associated with another material. The analysis ofthe phase data would be based on this knowledge instead of calculating11 and 12 from the measured data.

The present invention will be described with reference to five differentregions that may be measured on the surface of a substrate. A compositematerial with a transparent component will be treated like a transparentmaterial. The reference spot 102 and the measurement spot 104 may hitthe surface of the sample in five different combinations of locations,including:

1) one opaque material at two locations;

2) two different opaque materials;

3) one opaque and one transparent material;

4) one transparent material at two locations;

5) two different transparent materials.

Moreover, each of the above combinations may be measured in a referencemode or a differential mode.

One Opaque Material at Two Locations

FIG. 11 illustrates two beams orthogonal to the sample surfaceoriginating from an optical tool, e.g., a differential interferometer,measuring one opaque material 122 at two locations on a portion ofsample 120. As shown in FIG. 11, opaque material 122 resides, e.g.,directly on a substrate 123 or there may be materials between material122 and substrate 123. As shown in FIG. 11, the optical tool is using areference beam 124 and a measurement beam 126. FIG. 11 shows thedifferential interferometer operating in differential mode if the scandirection is parallel to the paper surface. Of course, the differentialinterferometer may operate in reference mode as well.

In this example, a referential scan is made. To simplify the analysis,the values in the measured phase profile (φ_(m)) are referenced to thefirst measurement point of the scan. The phase value from the firstmeasurement point (φ₀) is subtracted from every phase value at locationi (φ_(i)). The measured phase profile is made up of two or more valuesbased on the following equation:

φ_(m)=φ_(i)−φ₀  eq. 4

φ_(i) is the phase difference between the measurement spot and thereference spot at any measurement location i and φ₀ is the phasedifference between the measurement spot and the reference spot at thefirst point of the scan. At each location i, the initial phasemeasurement φ₀ is subtracted from the phase measurement at location i toyield the measured phase value. According to this procedure, the firstvalue of the measured phase profile is equal to zero.

Because the region of interest in FIG. 11 is composed of a single opaquematerial 122, such as a metal or metal alloy, no phase correction isrequired. The material specific phase shift associated with opaquematerial 122 is constant and thus can be ignored in this example. Themeasured phase profile can simply be converted to the actual heightprofile using the following formula: $\begin{matrix}{H_{c} = \frac{\phi_{m} \times \lambda}{4\quad \pi}} & {{eq}.\quad 5}\end{matrix}$

where H_(C) is the corrected height profile and λ is the wavelength oflight. By definition, H_(C) equals zero at the first measurement spot.Thus, the phase profile generated by the differential interferometer canbe directly converted to the actual height profile in reference mode, orcan be numerically integrated and converted to yield the actual heightprofile in differential mode. This procedure is commonly employed usinga differential interferometer.

Two Different Opaque Materials

FIG. 12 is similar to FIG. 11 but illustrates an optical tool, e.g., adifferential interferometer, measuring two different opaque materials132 and 134 on a portion of a sample 130. As shown in FIG. 12, opaquematerials 132 and 134 reside, e.g., directly on a substrate 133 or theremay be materials between substrate 133 and the opaque materials 132 and134. FIG. 12 shows the point in the scan in which the reference beam 124is incident on a first opaque material 132 while the measurement beam126 is incident on a second opaque material 134. FIG. 12 shows thedifferential interferometer operating in differential mode if the scandirection is parallel to the paper surface. Of course, the differentialinterferometer may operate in reference mode as well. It should beunderstood that when the reference beam 124 and measurement beam 126 areincident on a single opaque material, e.g., both are incident on opaquematerial 132 or both are incident on opaque material 134, the correctedheight profile would utilize the same procedure as described above inreference to FIG. 11.

FIG. 13 shows a flow chart 140 for generating the actual height profileof a reference scan of sample 130 with the measurement spot interceptingat least one point in both opaque materials 132 and 134 and thereference spot staying on opaque material 132 in FIG. 12. In step 141 amaterial specific phase measurement is made of the first opaque material132. In step 142 a material specific phase measurement is made of thesecond opaque material 134. The phase measurements may be made using,e.g., an ellipsometer, and will provide a material specific phase forthe material at one or more frequencies of radiation used by thedifferential interferometer. Alternatively, the phase measurements maybe obtained from a reference source, e.g., a library of phasemeasurements of different materials. Values for n and k are obtainedfrom the ellipsometer measurement or from a reference source andcombined to yield the material specific phase at a given frequency as isunderstood in the art and is discussed in detail in H. Jennewein, H.Gottschling, T. Ganz and T. Tschudi, “Interferometrical Profilometry atSurfaces with Varying Materials”, SPIE Conference on Metrology,Inspection and Process Control for Lithography XIII, Santa Clara,Calif., SPIE Vol. 3677, p. 1009-1016, March 1999, which is incorporatedherein by reference. Actually measuring the material with, e.g., anellipsometer, may provide a more accurate value of phase compared to areference source as n and k will likely vary with impurity level andsurface oxidation. Once the phase value for a material is obtained,either from a reference source or measured, the phase value may beplaced in a database for future reference.

The material specific phase difference between the first opaque material132 and second opaque material 134, i.e., the difference between thematerial specific phase values as measured or obtained from referencedata, is determined as follows (step 143):

Δφ_(O1,O2)=φ_(O1)−φ_(O2)  eq. 6

where φ_(O1) is the material specific phase value of the first opaquematerial 132, φ_(O2) is the material specific phase value of secondopaque material 134, and Δφ_(O1,O2) is the material specific phasedifference for opaque materials 132 and 134.

A measurement is then made in reference mode with an optical tool, e.g.,a differential interferometer that employs one or more wavelengths oflight (step 144). In a typical scan, the measurement starts with bothspots residing in opaque area 132, followed by the measurement spottraversing opaque material 134 followed by both spots residing in opaquematerial 132. As discussed above, in reference mode, the reference spotis incident on two or more points, a line or an area on material 132that remain at a constant height and does not travel over opaque area134.

Similar to the previous example, the phase profile is referenced to thefirst point of the scan. The measured phase profile is made up of two ormore values based on the following equation:

φ_(m)=φ_(i)−φ₀  eq. 7

At each location i, the initial phase measurement φ₀ is subtracted fromthe phase measurement at location i to yield the measured phase value.The portions of the measured phase profile (φ_(m)) when the measurementspot is incident on opaque material 132 do not need to be corrected, theactual phase profile (φ_(c)) is equal to the measured phase profile:

 φ_(c)=φ_(m)  eq. 8

When the measurement spot is incident on opaque material 134 (and thereference spot is incident on opaque material 132), the phase profile iscorrected using the material specific phase difference from equation 6(step 145):

φ_(c)=φ_(m)−Δφ_(O1,O2)  eq. 9

The corrected portion of the measured phase profile when the measurementspot is incident on opaque material 134 along with the portions of thephase profile where the measurement spot was incident on opaque material132 yield the corrected or actual phase profile of the entire scan. Theactual phase profile can be converted to the actual height profile(H_(c)) using the following formula (step 146): $\begin{matrix}{H_{c} = \frac{\phi_{c} \times \lambda}{4\quad \pi}} & {{eq}.\quad 10}\end{matrix}$

A number of measurements may be extracted from the actual height profileto generate a measurement such as dishing, erosion, curvature, or otherdesired measurements (step 147).

If a region can be found on the sample where there is no step heightdifference between the two opaque materials, a direct measurement of thematerial specific phase difference between the two materials can bemade, for example, using a differential interferometer, therebyreplacing steps 141 and 142. First a measurement must be made with bothspots hitting the same opaque material at the same height. Then ameasurement is made with one spot on each opaque material at the sameheight. The difference between these two measurements yields thematerial specific phase difference between the two materials (step 143).Alternatively, if a region can be found on the sample where there is aknown step height difference between the two opaque materials, ameasurement of the phase difference between the two materials can bemade as above, e.g., using a differential interferometer, aftersubtracting the contribution of the known step height.

FIG. 14 shows a flow chart 150 for generating the actual height profileof sample 130 using a differential scan with the measurement spotintercepting at least one point in each opaque material 132 and 134shown in FIG. 12. The method of generating the actual height profileusing a differential scan is similar to the method of using a referencescan as described in FIG. 13. As shown in FIG. 14, in step 151 amaterial specific phase measurement is made of the first opaque material132. In step 152 a material specific phase measurement is made of thesecond opaque material 134.

The material specific phase difference between the first opaque material132 and second opaque material 134, i.e., the difference between thematerial specific phase values as measured or obtained from referencedata, is determined (step 153) as follows:

Δφ_(O1,O2)=φ_(O1)−φ_(O2)  eq. 11

where φ_(O1) is the material specific phase value of the first opaquematerial 132, φ_(O2) is the material specific phase value of secondopaque material 134, and Δφ_(O1,O2) is the material specific phasedifference for opaque materials 132 and 134. A similar parameterΔφ_(O2,O1) must also be calculated for use when the materials that thespots intercept are reversed. The parameter Δφ_(O2,O1) is simply thenegative of Δφ_(O1,O2).

A measurement is then made in differential mode with an optical tool,e.g., a differential interferometer that employs one or more wavelengthsof light (step 154). In a typical scan, the measurement starts with bothspots residing in opaque area 132, followed by the measurement spottraversing opaque material 134 while the reference spot is still inopaque material 132, followed by both spots residing in opaque material134, followed by the measurement spot traversing opaque material 132while the reference spot is still in opaque material 134 and finallyfollowed by both spots residing in opaque material 132. This scenarioassumes that the spacing of the two spots is smaller than the dimensionof the feature made of opaque material 134. If the spacing of the twospots is larger than the dimension of the feature made of opaquematerial 134, both spots would never simultaneously reside in thefeature made of opaque material 134. Although with a small enough pitchand a large spot spacing, the spots could both reside in two differentfeatures made up of opaque material 134.

With a differential mode scan, the phase profile must be numericallyintegrated to yield the phase profile that would be generated by areferential scan (step 155).

The portions of the measured phase profile (φ_(m)) when both spots areincident on opaque material 132 do not need to be corrected, thecorrected or actual phase profile (φ_(c)) is equal to the measured phaseprofile:

φ_(c)=φ_(m)  eq. 12

When either spot is incident on opaque material 134, the phase profile(φ_(m)) is corrected using one of two material specific phasedifferences depending on the materials intercepted by the two spots(step 156):

φ_(c)=φ_(m)−Δφ_(O1,O2)  eq. 13

φ_(c)=φ_(m)−Δφ_(O2,O1)  eq. 14

The corrected portion of the measured phase profile when either spot isincident on opaque material 134 along with the portions of the phaseprofile where both measurement spots are incident on opaque material 132yield the corrected or actual phase profile of the entire scan.

The actual-phase profile can be converted to the actual height profileusing the following formula (step 157): $\begin{matrix}{H_{c} = \frac{\phi_{c} \times \lambda}{4\quad \pi}} & {{eq}.\quad 15}\end{matrix}$

A number of measurements may be extracted from the actual height profileto generate a measurement such as dishing, erosion, curvature, or otherdesired measurements (step 158).

If a region can be found on the sample where there is no step heightdifference between the two opaque materials, a direct measurement of thematerial specific phase difference between the two materials can bemade, for example, using a differential interferometer, therebyreplacing steps 151 and 152. The difference between these twomeasurements yields the material specific phase difference between thetwo materials (step 153). Alternatively, if a region can be found on thesample where there is a known step height difference between the twoopaque materials, a measurement of the phase difference between the twomaterials can be made as above, e.g., using a differentialinterferometer, after subtracting the contribution of the known stepheight.

One Opaque Material and One Transparent Material

The process of depositing copper into trenches and vias formed on adielectric substrate followed by polishing of the-copper may result indishing and erosion as discussed in reference to FIGS. 2A, 2B, 2C, 3 and4. The resulting dished and eroded regions may be accurately measured inaccordance with the present embodiment.

FIG. 15 is similar to FIG. 12 but illustrates an optical tool, e.g., adifferential interferometer, measuring the height profile of an opaquematerial 164 and a transparent material 162 on a portion of a sample160. The transparent material 162 resides on opaque or extremely thicksubstrate 166. The reference beam 124 is incident on the transparentmaterial 162, which may be a dielectric layer or stack, e.g., anoxide/nitride/oxide stack, while the measurement beam 126 is incident onthe opaque material 164, which may be, e.g., copper, aluminum ortungsten or alloys thereof.

Again, while FIG. 15 shows the differential interferometer operating indifferential mode if the scan direction is parallel to the papersurface, it should be understood that the differential interferometermay operate in referential mode with the reference beam 124 beingincident on an opaque material, similar to material 164 or ontransparent material 162. The starting location must be in an area ofconstant height or thickness for a differential mode measurement or twoor more reference points, a line or an area must be at the same heightor thickness for a referential mode measurement. Moreover, it should beunderstood that when the reference beam 124 and measurement beam 126 areboth incident on the same opaque material, e.g., both are incident onopaque material 164, the measurement of the height profile is the sameas described above in reference to FIG. 11.

FIG. 16 shows a flow chart 170 for generating the actual height profileof a reference scan of sample 160 with the measurement spot interceptingat least one point in opaque material 164 and one point in transparentmaterial 162 and the reference spot staying in transparent material 162in FIG. 15. In step 171 a material specific phase and thicknessmeasurement is made of transparent material 162 preferably at thestarting location of the measurement sport using, e.g., a reflectometeror ellipsometer. The thickness measurement of transparent material 162at this location will provide the absolute thickness of the actualthickness profile and may be necessary if material 162 has more than twoor three thickness values for a particular phase value. This measurementis experimentally determined because of the variation of phase withthickness. All reference points, a reference line or a reference areamust be at the same thickness for a precise reference mode measurement.

The relationship between the thickness and phase of the transparentmaterial 162 is generated for the thickness range of interest (step172). A simplification is to assume that only the top layer of amulti-layer dielectric stack is changing in thickness over the distanceof the scan. Local erosion of the dielectric stack from the CMP processallows this assumption to be made. For example, if the measuredthickness of transparent material 162 is 1.0 μm, the relationshipbetween the thickness and phase would be generated from 0.7 μm to 1.1 μmwith the change in thickness resulting from the uppermost layer oftransparent material 162. FIG. 18 shows a plot 192 of the variation ofthe phase of the entire transparent material or stack 162 over athickness range with only the top layer varying in thickness. As can beseen, the phase of the transparent material is not linear as thethickness increases. Thus, a mathematical model must be made to generatethe relationship between thickness and phase of the transparent materialor stack 162. This relationship is a complex function of thethicknesses, indices of refraction and extinction coefficients of thedielectric stack materials and is generated by adding the contributionsof the waves reflected and transmitted at each interface by using ageneralized form of the Fresnel equations. Because the relationshipbetween the thickness and phase is not always monotonic, it may benecessary to measure the thickness of the transparent material 162 atthe starting location in step 171, for example, in the situation wherethere are two or three thickness values for a particular phase value.

The phase versus thickness relationship can be extracted from thegeneralized complex reflection coefficients r_(s) and r_(p). Thesecoefficients for s- and p-polarizations of a film stack are given by arecursion procedure to the following two formulas for each film in thedielectric stack: $\begin{matrix}{r_{s} = \frac{r_{{s\quad j},j} + 1 + r_{{s\quad j},j} - {1\quad {\exp \left( {{- i}\quad 2\quad \beta_{j}} \right)}}}{1 + r_{{s\quad j},j} + {1\quad r_{{s\quad j},j}} - {1\quad {\exp \left( {{- i}\quad 2\quad \beta_{j}} \right)}}}} & {{{eq}.\quad 16}a} \\{r_{p} = \frac{r_{{p\quad j},j} + 1 + r_{{p\quad j},j} - {1\quad {\exp \left( {{- i}\quad 2\quad \beta_{j}} \right)}}}{1 + r_{{p\quad j},j} + {1\quad r_{{p\quad j},j}} - {1\quad {\exp \left( {{- i}\quad 2\quad \beta_{j}} \right)}}}} & {{{eq}.\quad 16}b}\end{matrix}$

where the following two equations correspond to the standard Fresnelcoefficients at the interface between layer i and j: $\begin{matrix}{r_{s\quad i\quad j} = \frac{{n_{i}\cos \quad \theta_{i}} - {n_{j}\cos \quad \theta_{j}}}{{n_{i}\cos \quad \theta_{i}} + {n_{j}\cos \quad \theta_{j}}}} & {{{eq}.\quad 17}a} \\{r_{p\quad i\quad j} = \frac{{n_{i}\cos \quad \theta_{j}} - {n_{j}\cos \quad \theta_{i}}}{{n_{i}\cos \quad \theta_{j}} + {n_{j}\cos \quad \theta_{i}}}} & {{{eq}.\quad 17}a}\end{matrix}$

and β_(j) is the phase shift caused by film j upon reflection and isdefined as: $\begin{matrix}{\beta_{j} = \frac{2\quad \pi \quad n_{j}\cos \quad \theta_{i}t_{j}}{\lambda}} & {{eq}.\quad 18}\end{matrix}$

where n_(j) is the refractive index of film j, θ_(j) is the angle ofincidence of the measurement beam, t_(j) is the thickness of film j andλ is the wavelength of light. Equations 16a, 16b, 17a, 17b and 18 arefrom R. M. A. Azzam and N. M. Bashara, “Ellipsometry and PolarizedLight”, Elsevier, Amsterdam, 1999, which is incorporated herein byreference. The mathematical procedure to generate the relationshipbetween thickness and phase is also described in Gee Hong Kim and SeungWoo Kim, “White light scanning interferometry for thickness measurementof thin film layers”, SPIE Conference on Optical Diagnostics forFluids/Heat/Combustion and Photomechanics for Solids, Denver Colo., SPIEVol. 3783, p. 239, July 1999; and O. S. Heavens, “Optical properties ofthin solid films”, Dover Publications Inc., Mineola, N.Y., 1991, whichare incorporated herein by reference.

When a composite material with a transparent component is involved, asimilar procedure is employed to generate the phase versus thicknessrelationship. However, the generation of the phase versus thicknessrelationship for a composite material has a somewhat greater complexitythan for a simple transparent material due to the additional materialcomponents and the variables related to the specific geometry associatedwith the composite material. Due to the nature of the compositematerial, a model must be developed to predict the variation of phasewith thickness when a constant phase opaque material is illuminatedalong with a transparent material. The opaque feature density and thepitch will affect the output of the model and typically results in arelationship similar to that of a phase versus thickness relationshipfor a simple transparent material but exhibits a smaller average slopedue to the constant phase behavior of the opaque material. A simplemodel of the phase versus thickness relationship for a composite region,by way of example, might weight the varying phase of the transparentmaterial and the constant phase of the opaque material by the metaldensity while ignoring the pitch. In step 173, a material specific phasemeasurement is made of opaque material 164. The phase measurements maybe made using, e.g., an ellipsometer, and will provide a materialspecific phase for the material at one or more frequencies used by thedifferential interferometer. Alternatively, the phase measurements maybe obtained from a reference source, e.g., a library of phasemeasurements of different materials, as discussed above.

The material specific phase difference between the transparent material162 and the opaque material 164, is calculated (step 174) as follows:

Δφ_(T,O)=φ_(T)−φ_(O)  eq. 19

where φ_(T) is the material specific phase value of the transparentmaterial 162 at the starting measurement location of the scan, φ_(O) isthe material specific phase value of opaque material 164, and Δφ_(T,O)is the material specific phase difference between the transparentmaterial 162 and the opaque material 164.

A measurement is then made in reference mode with an optical tool, i.e.,a differential interferometer that employs one or more wavelengths oflight (step 175). In a typical scan, the measurement starts with bothspots residing in transparent material 162, followed by the measurementspot traversing opaque material 164 followed by both spots residing intransparent material 162. In the reference mode, the reference spot ispreferably incident on two or more points, a line or an area oftransparent material 162 that remains at a constant thickness and doesnot travel over opaque material 164.

When the measurement spot is incident on opaque material 164, the phaseprofile is corrected using the material specific phase difference fromequation 19 (step 176):

φ_(c)=φ_(m)−φ_(T,O)  eq. 20

When the measurement spot is incident on opaque material 164, the phaseprofile is converted to a height profile using the following equation(step 177): $\begin{matrix}{H_{c} = \frac{\phi_{c} \times \lambda}{4\quad \pi}} & {{eq}.\quad 21}\end{matrix}$

The material specific phase (and thickness if necessary) of transparentmaterial 162 and the relationship between the thickness and phase oftransparent material 162 from step 172 are used, advantageously, toconvert the phase profile to a height profile when both the referencebeam 124 and the measurement beam 126 are incident on transparentmaterial 162 (step 178). This is particularly useful to measure erosionof the transparent material that is near the opaque material. FIG. 19shows the particular area of interest 194 of sample 160 where thetransparent material or stack 162 residing over the substrate 166 erodesnear the opaque material 164.

The material specific phase reflectometer or ellipsometer measurementfrom step 171 is located on the plot from step 172 (FIG. 18). Thethickness of the transparent material at this location can be read fromthis plot. If the relationship between thickness and phase for thetransparent material is such that there are two or three thicknessvalues for a particular phase value, the thickness measurement from step171 must be used to identify the correct thickness in FIG. 18. The phasevalue from step 171 is then subtracted from all of the followinginterferometer phase measurements. The converted measurements nowrepresent the difference in phase of each measurement point with respectto the first interferometer point that is coincident with thereflectometer or ellipsometer measurement location. By definition, thedifference in phase value is zero for the first measurement point. Forall of the other points, the difference in phase measurement can now belocated on the y axis with respect to the first measurement phase valueand the thickness can be read off of the x axis of FIG. 18. Once all ofthe phase measurement points are converted to thickness, the actualheight profile of the transparent region can be generated.

A number of measurements may be extracted from the actual height profileto generate a measurement such as dishing, erosion, curvature, or otherdesired measurements (step 179).

If a region can be found on the sample where there is no step heightdifference between the transparent material and the opaque material, adirect measurement of the material specific phase difference between thetwo materials can be made thereby replacing step 173. First the phasedifference between the transparent and opaque materials with no step ismeasured, e.g., using a differential interferometer. Next, the phasedifference between the transparent material at this location and thetransparent material at the starting measurement location is measured,e.g., using a differential interferometer. The material specific phasedifference is calculated from these two measurements (step 174). Themeasured height profile can then be corrected (step 176) using thisphase difference, rather than the standard calculated phase difference.Alternatively, if a region can be found on the sample with a known stepheight between the transparent material and the opaque material, ameasurement of the phase difference between the two materials can bemade, e.g., using a differential interferometer, after subtracting thecontribution of the known step height along with the phase differencebetween the transparent material at this location and the transparentmaterial at the starting measurement location. The measured heightprofile can again be corrected using this measured phase difference.

In accordance with another embodiment of the present invention, adishing value may be obtained without generating the actual heightprofile for the entire scan. This embodiment is similar to theembodiment described in reference to FIG. 16, however steps 172 and 178are omitted. The relationship between thickness and phase for thetransparent material is not generated and the phase measurements of thetransparent regions are not converted to thickness. The location of thefirst measurement spot of the differential interferometer scan coincideswith the location of the material specific phase and thicknessmeasurement. It is assumed that this point represents the maximumthickness of the transparent region, which is a reasonable assumptionbecause this point occurs in an area that is supposed to be flat and is,therefore, close to the highest point of the transparent area. Theabbreviated procedure corrects the regions of the phase profile when themeasurement spot hits the opaque material 164 (step 176) using thematerial specific phase difference from steps 171, 173 and 174. Then itconverts the phase profile to a height profile for these opaque material164 regions (step 177). This yields a height profile that displays theaccurate relationship of the opaque regions with respect to the firstmeasurement point of the scan. It does not accurately portray thethickness profile in the transparent regions, the erosion areas adjacentto the opaque feature. After converting the phase profile to a heightprofile, dishing can be calculated (step 179). This particulardefinition of dishing is the difference in height between a defined lowarea of a portion or all of an opaque material 164 region and the heightof the transparent material 162 at the first measurement spot location.

Moreover, a dishing value may be obtained by generating the actualheight profile for only a portion of the opaque material 164 from FIG.15. For example, when the measurement spot hits the opaque material 164(step 176), the material specific phase difference from steps 171, 173and 174 may be used to correct the phase profile for only the lowestportion of the opaque material 164. The phase profile for the lowestportion of the opaque material is then converted to a height profile(step 177). This yields a height profile that displays the accuraterelationship of only the lowest portion of the opaque region withrespect to the first measurement point of the scan or any chosen portionof the transparent material 162. In one embodiment, for example, it maybe desirable to locate a fraction of the lowest measurement points, 1%to 50%, e.g., 20%, of an opaque material 164 region and use the averageof these values to represent the lowest value of this region. Forexample, intensity information could be used to locate the center 20% ofthe opaque region that is then chosen as the lowest portion of theopaque region. A simple dishing value can then be calculated bysubtracting the chosen low point or average of points in the opaquematerial 164 from the height associated with the first point of the scanor any chosen point or average of points in transparent material 162. Inaddition, it should be noted that there are a number of ways todetermine the amount of dishing. For example, the height of thetransparent feature may be defined far from the opaque feature whereerosion is minimal or near the opaque feature where there is significanterosion. Another option is to average the height of the two transparentfeatures surrounding the opaque feature to yield an average transparentregion height.

FIG. 17 shows a flow chart 180 for generating the actual height profileof a differential scan of sample 160 with the measurement spotintercepting at least one point in transparent material 162 and opaquematerial 164 in FIG. 15. It is very similar to the method used with areference scan described in FIG. 16. In step 181, a material specificphase and thickness measurement is made of transparent-material 162preferably at the starting location of the measurement spot. A thicknessmeasurement of transparent material 162 at this location, if performed,will provide the absolute height of the actual height profile.Preferably, the measurement is started at a location in transparentmaterial 162 that is at a constant height so that both spots are at thesame phase value.

The relationship between the thickness and phase of the transparentmaterial 162 is generated for the thickness range of interest (step182).

In step 183, a material specific phase measurement is made of opaquematerial 164. The phase measurements may be made using, e.g., anellipsometer, and will provide a material specific phase for thematerial at one or more frequencies used by the differentialinterferometer. Alternatively, the phase measurements may be obtainedfrom a reference source, e.g., a library of phase measurements ofdifferent materials.

The material specific phase difference between the transparent material162 and the opaque material 164, is calculated (step 184). The sign ofthis phase difference will change when the measurement and referencespots swap the intercepted materials.

A measurement is then made in differential mode with an optical tool,i.e., a differential interferometer that employs one or more wavelengthsof light (step 185). In a typical scan, the measurement starts with bothspots residing in transparent material 162, followed by the measurementspot traversing opaque material 164 while the reference spot is still inthe transparent material 162, followed by both spots residing in opaquematerial 164, followed by the measurement spot traversing transparentmaterial 162 while the reference spot is still in opaque material 164and finally followed by both spots residing in transparent material 162.This scenario assumes that the spacing of the two spots is smaller thanthe dimension of the opaque feature.

With a differential mode scan, the phase profile must be numericallyintegrated to yield the phase profile that would be generated by areferential scan (step 186).

When either spot is incident on opaque material 164, the phase profileis corrected using one of two material specific phase differences (step187).

This corrected phase profile is then converted to a height profile whenthe measurement spot hits the opaque material 164 (step 188).

The material specific phase and thickness of transparent material 162from step 181 and the relationship between the thickness and phase oftransparent material 162 from step 182 are used, advantageously, toconvert the phase profile to a thickness profile when the measurementbeam 126 is incident on transparent material 162 (step 189). Theprocedure is the same as described previously.

A number of measurements may be extracted from the actual height profileto generate a measurement such as dishing, erosion, curvature,curvature, or other desired measurements (step 190).

If a region can be found on the sample where there is no step heightdifference between the transparent material and the opaque material, adirect measurement of the material specific phase difference between thetwo materials can be made thereby replacing step 183. First, the phasedifference between the transparent and opaque materials with no step ismeasured, e.g., using a differential interferometer. Next, the phasedifference between the transparent material at this location and thetransparent material at the starting measurement location is measured,e.g., using a differential interferometer. The material specific phasedifference is calculated from these two measurements (step 184). Themeasured height profile can then be corrected (step 187) using thisphase difference, rather than the standard calculated phase difference.Alternatively, if a region can be found on the sample with a known stepheight between the transparent material and the opaque material, ameasurement of the phase difference between the two materials can bemade, e.g., using a differential interferometer, after subtracting thecontribution of the known step height along with the phase differencebetween the transparent material at this location and the transparentmaterial at the starting measurement location. The measured heightprofile can again be corrected using this measured phase difference.

In accordance with another embodiment of the present invention, adishing value may be obtained without generating the actual heightprofile for the entire scan. This embodiment is similar to theembodiment described in reference to FIG. 17, however steps 182 and 189are omitted. The relationship between thickness and phase for thetransparent material is not generated and the phase measurements of thetransparent regions are not converted to thickness. The location of thefirst measurement spot of the DI scan coincides with the location of thematerial specific phase and thickness measurement. It is assumed thatthis point represents the maximum thickness of the transparent region,which is a reasonable assumption because this point occurs in an areathat is supposed to be flat and is, therefore, close to the highestpoint of the transparent area. The abbreviated procedure corrects theregions of the phase profile when each spots hits a different material(step 187) using the material specific phase difference from steps 181,183 and 184. Then it converts the phase profile to a height profile forthese opaque material 164 regions (step 188). This yields a heightprofile that displays the accurate relationship of the opaque regionswith respect to the first measurement point of the scan. It does notaccurately portray the thickness profile in the transparent regions.After converting the phase profile to a height profile, dishing can becalculated (step 190). This particular definition of dishing is thedifference in height between a defined low area of a fraction or all ofan opaque material 164 region and the height of the transparent material162 at the first measurement point location.

One Transparent at Two Locations

The actual thickness profile of a region containing one transparent orcomposite material, e.g., a dielectric material or stack, of changingthickness, e.g., region 194 shown in FIG. 19, will be described. It willbe a subset of the flow chart described in FIG. 16 for a reference scanor a subset of the flow chart described in FIG. 17 for a differentialscan.

For simplicity, it is assumed that substrate 166 is substantially planarbelow transparent material 162 so that the calculated changes inthickness will be assumed to be entirely due to transparent material162. The reference mode measurement of an actual thickness profile of aregion is similar to that described in flow chart 170 in FIG. 16, wherethe material specific phase and thickness of transparent material 162 ismeasured at the starting measurement spot location (step 171). Asdiscussed above, the thickness measurement may be necessary if thetransparent material has the same material specific phase value at twoor three thicknesses. The relationship between the thickness and phaseof the transparent material 162 is generated for the thickness range ofinterest (step 172). A reference mode measurement is made of thetransparent material 162 oyer region 194 in FIG. 19 to generate thephase profile (step 175). Because there is no opaque material in thisembodiment, steps 173, 174, 176 and 177 are skipped. The materialspecific phase and thickness of transparent material 162 from step 171and the relationship between the thickness and phase of transparentmaterial 162 from step 172 are used, advantageously, to correct thethickness profile when both the reference beam 124 and the measurementbeam 126 are incident on transparent material 162 (step 178). Forexample, one use of this actual transparent material thickness profileis to measure erosion of the transparent material that is near an opaquematerial structure (step 179).

The differential mode measurement of an actual thickness profile of aregion is similar to that described in flow chart 180 in FIG. 17, wherethe material specific phase and thickness of transparent material 162 ismeasured at the starting measurement spot location (step 181). Therelationship between the thickness and phase of the transparent material162 is generated for the thickness range of interest (step 182). Adifferential mode measurement is made of the transparent material 162over region 188 in FIG. 19 to generate the phase profile (step 185). Thephase profile is numerically integrated to yield the same phase profilethat would be generated with a referential scan (step 186). Becausethere is no opaque material in this embodiment, steps 183, 184, 187 and188 are skipped. The material specific phase and thickness oftransparent material 162 from step 181 and the relationship between thethickness and phase of transparent material 162 from step 182 are used,advantageously, to correct the thickness profile when both the referencebeam 124 and the measurement beam 126 are incident on transparentmaterial 162 (step 189). This actual thickness profile is used tomeasure erosion of the transparent material that is near the opaquematerial (step 190).

Two Different Transparent Materials

The following discussions are equally applicable for samples thatcontain two transparent materials, one transparent and one compositematerial or two composite materials. The only significant differencesare the models used to generate the relationship between thickness andphase for a composite material compared to a transparent material. Theshape of the thickness versus phase curve for a composite material willbe similar to the curve for a transparent material as shown in FIG. 18.The case of one transparent and one composite material is of interestbecause of the erosion problem associated with a copper CMP process. Inthe following example, the first transparent material is composed of adielectric stack and the second transparent material is a compositematerial composed of a dielectric stack and embedded opaque metal lines.The pitch associated with the metal lines is smaller than the spot sizeassociated with the differential interferometer. The presence of themetal lines enhances the composite material CMP rate, thus creatingerosion.

FIG. 20 illustrates two beams orthogonal to the sample surfaceoriginating from an optical tool, e.g., a differential interferometer,measuring the thickness profile of transparent material 202 andcomposite material 204 on a portion of a sample 200. Both thetransparent material 202 and the composite material 204 are assumed toreside on opaque or extremely thick substrate 210. The reference beam124 is shown incident on the transparent material 202, which may be adielectric layer or stack, e.g., an oxide/nitride/oxide stack, while themeasurement beam 126 is shown incident on the composite material 204.For example, as shown in FIG. 20, composite material 204 may includedensely packed opaque lines 206 of copper or other metal/metal alloy andtransparent material 208, which may be the same as transparent material202. Thus, the actual thickness profile may be used to measure erosionof a region with a specified metal density and pitch on an area of asemiconductor sample. FIG. 21 shows a measurement spot 209 incident oncomposite material 204. As shown in FIG. 21, opaque lines 206 have asmall width and thus spot 209 overlaps several lines. Consequently,composite material 204 is partially transparent and partially opaque.For simplicity, it is assumed that substrate 210 is substantially planarbelow transparent material 202 and composite material 204 so that thecalculated changes in thickness will be assumed to be entirely due totransparent material 202 and composite material 204 and not substrate210.

Again, while FIG. 20 shows the differential interferometer operating indifferential mode if the scan direction is parallel to the papersurface, it should be understood that the differential interferometermay operate in reference mode. The starting measurement location must bein an area of constant thickness for a differential mode measurement orboth reference points, the reference line or the reference area must beat the same thickness for a referential mode measurement.

FIGS. 22A and 22B show a flow chart 210 for generating the actualthickness profile of a reference scan of sample 200 with the measurementspot intercepting at least one point in transparent material 202 and onepoint in composite material 204 and the reference spot staying intransparent material 202 in FIG. 20. It also assumes that a phase andthickness measurement will be made for each material. In step 211, amaterial specific phase and thickness measurement is made of transparentmaterial 202 preferably at the starting measurement spot location. Thethickness measurement of transparent material 202 at this location willprovide the absolute thickness of the actual thickness profile and maybe necessary if material 202 has two or three thickness values for aparticular phase value. All reference points, a reference line or areference area must be at the same thickness for a precise referentialmode measurement.

In step 212, a material specific phase and thickness measurement is madeof composite material 204 at a second location that must coincide with ameasurement point of the differential interferometer.

The relationship between the thickness and phase of the transparentmaterial 202 is generated for the thickness range of interest (step 213)as well as the relationship between the thickness and phase of thecomposite material 204 (step 214).

The procedure can optionally be stopped at this step if only adifference in thickness between these two measurement locations and notan actual thickness profile is desired. This difference in phase can beconverted to a difference in thickness or the difference in thicknessescan be directly calculated to yield the difference in thickness betweenthe two measurement locations. No differential interferometermeasurement is required if only a difference in thickness is required.

A measurement is then made in reference mode with an optical tool, e.g.,a differential interferometer that employs one or more wavelengths oflight (step 216). In a typical scan, the measurement starts with bothspots residing in transparent material 202, followed by the measurementspot traversing composite material 204 followed by both spots residingin transparent material 202. In the reference mode, the reference spotis preferably incident on two or more points, a line or an area oftransparent material 202 that remains at a constant thickness and doesnot travel over opaque material 204.

The material specific phase and thickness of transparent material 202from step 211 and the relationship between the thickness and phase oftransparent material 202 from step 213 are used, advantageously, toconvert the phase profile to a thickness profile when both the referencebeam 124 and the measurement beam 126 are incident on transparentmaterial 202 (step 218).

The material specific phase and thickness of composite material 204 fromstep 212 and the relationship between the thickness and phase ofcomposite material 204 from step 214 are used, advantageously, toconvert the phase profile to a thickness profile when the measurementbeam 126 is incident on composite material 204 (step 219).

A number of measurements may be extracted from the actual thicknessprofile to generate a measurement such as erosion, curvature, or otherdesired measurements (step 220).

If an accurate measurement of the erosion of transparent material 202near the location of a feature composed of composite material 204 is notdesired, it is not necessary to generate the relationship between thethickness and phase of the transparent material 202 (step 213) orcorrect the phase profile when both spots hit the transparent material(step 218).

FIGS. 23A and 23B show a flow chart 230 for generating the actualthickness profile of a differential scan of sample 200 with themeasurement spot intercepting at least one point in transparent material202 and composite material 204 in FIG. 20. It also assumes that a phaseand thickness measurement will be made for each material. Steps 231 and232 may be required, for example, if the model used to generate thethickness versus phase for a composite material shows poor precisioncapability.

In step 231, a material specific phase and thickness measurement is madeof transparent material 202 preferably at the starting measurement spotlocation. The thickness measurement of transparent material 202 at thislocation will provide the absolute thickness of the actual thicknessprofile and may be necessary if there are two or three thicknesses for aparticular phase value. Preferably, the measurement is started at alocation in transparent material 202 that is at a constant thickness sothat both spots are at the same phase value.

In step 232, a material specific phase and thickness measurement is madeof composite material 204 at a second location that must coincide with ameasurement point of the differential interferometer. Again, thethickness value may need to be measured at the second location if thecomposite material has two or three thicknesses for a particular phasevalue.

The relationship between the thickness and phase of the transparentmaterial 202 is generated for the thickness range of interest (step 233)as well as the relationship between the thickness and phase of thecomposite material 204 (step 234).

The material specific phase difference between the transparent material202 at the measurement spot starting location and the composite material204 at the second location is calculated (step 235). The procedure canoptionally be stopped at this step if only a difference in thicknessbetween these two measurement locations and not an actual thicknessprofile is desired.

A measurement is then made in differential mode with an optical tool,e.g., a differential interferometer that employs one or more wavelengthsof light (step 236). In a typical scan, the measurement starts with bothspots residing in transparent material 202, followed by the measurementspot traversing composite material 204 while the reference spot is stillin transparent material 202, followed by both spots residing incomposite material 202, followed by the measurement spot traversingtransparent material 202 while the reference spot is still in compositematerial 204 and finally followed by both spots residing in transparentmaterial 202. This scenario assumes that the spacing of the two spots issmaller than the dimension of the composite feature.

With a differential mode scan, the phase profile must be numericallyintegrated to yield the phase profile that would be generated by areferential scan (step 237).

When one spot is incident on transparent material 202 and one spot isincident on composite material 204, the phase profile is corrected usingone of two material specific phase differences (step 238).

The material specific phase and thickness of transparent material 202from step 231 and the relationship between the thickness and phase oftransparent material 202 from step 233 are used, advantageously, toconvert the phase profile to a thickness profile when the measurementbeam 126 is incident on transparent material 202 (step 239).

The material specific phase and thickness of transparent material 204from step 232 and the relationship between the thickness and phase ofcomposite material 204 from step 234 are used, advantageously, toconvert the phase profile to a thickness profile when the measurementbeam 126 is incident on composite material 204 (step 240).

A number of measurements maybe extracted from the actual thicknessprofile to generate a measurement such as erosion, curvature, or otherdesired measurements (step 241).

If an accurate measurement of the erosion of transparent material 202near the location of a feature composed of composite material 204 is notdesired, it is not necessary to generate the relationship between thethickness and phase of the transparent material 202 (step 233) orcorrect the thickness profile when both spots hit the transparentmaterial (step 239).

Although the present invention is illustrated in connection withspecific embodiments for instructional purposes, the present inventionis not limited thereto. Various adaptations, modifications, andcombinations may be made without departing from the scope of theinvention. Moreover, it should be understood that a metrology processmay be used with wafers, flat panel displays or any other device inwhich the measurement of the thickness profile, including dishing,erosion and/or curvature is desirable. Further, it should be understoodthat the data may be stored in a computer readable medium andmanipulated mathematically using, e.g., an appropriate processor ormicroprocessor reading software, which may be written by one of ordinaryskill in the art in light of the present disclosure. As discussed, onlyfraction of the data need be converted to an actual height profile toproduce a meaningful measurement of dishing or erosion. Therefore, thespirit and scope of the appended claims should not be limited to theforegoing description.

What is claimed is:
 1. A method of determining the material interceptedby a spot of light during the measurement of a region including at leasta first material and a second material, said method comprising: scanninga measurement spot of light from a differential interferometer acrosssaid region; measuring the intensity of at least said measurement spotof light scanning across said region; and determining the material thatat least said measurement spot of light is incident upon by the measuredintensity.
 2. The method of claim 1, further comprising: modulating thephase of radiation produced by a laser; splitting said radiationproduced by said laser into two beams; reflecting said two beams of saidsample; recombining said two beams into a single beam; and detectingsaid single beam.
 3. The method of claim 2, wherein detecting saidsingle beam produces a detector signal, said method further comprising:analyzing the intensity of said detector signal usingIntensity=A+B×Cos(CV+D) where A is the average intensity of the detectorsignal, B is one half of the peak to peak intensity, C is the modulatorsensitivity, V is the applied modulator voltage and D is the phase shiftassociated with the two beams.
 4. The method of claim 3, furthercomprising: determining a first intensity of one of said two beams and asecond intensity of the other of said two beams using A and B and usingthe following formulas:${{First}\quad {Intensity}} = \frac{A + \sqrt{A^{2} - B^{2}}}{2}$${{{Second}\quad {Intensity}} = \frac{A - \sqrt{A^{2} - B^{2}}}{2}};$

and determining the material that said two beams are incident upon usingat least one of said first intensity and said second intensity.